MARCO TEÓRICO
CLIMA ORGANIZACIONAL:
The term "soft salt" describes ionic materials that are composed of two or more organometal- lic components characterised by complementary charges and assembled into complex salts through Coulombic interactions. The majority of the reported soft salts incorporating iridium have consisted of a cationic iridium complex married with an anionic iridium
complex.
The first examples of iridium soft salts were reported in 2010 by Thompson and co-workers[71] (Figure 4) and De Cola and co-workers[72] (Figure 5a) who assembled via salt metathesis reactions blue- and green-emitting anionic Ir(III) complexes with yellow- and orange-emitting cationic iridium complexes.
Figure 4. Chemical structure of soft salt S1(left) and S2(right).
Soft saltS1was prepared by assembling the anionic green-emitting Ir(III) complex Na[Ir(mppyH)2(CN)2], Na1awith the cationic yellow-emitting Ir(III) complex [Ir(mppyH)2 (CNdt)2]Cl, 1bCl(mppyH is 2-(p-tolyl)pyridine and CNdt is 2-methyl-N-methylydynepropan-
2-aminium), whileS2contains the anionic blue-emitting Ir(III) complex Na[Ir(dFppy)2(CN)2], Na2a (dFppyH is 4,6-difluorophenylpyridine) with the cationic orange-emitting Ir(III) com- plex [Ir(mppyH)2(dtBubpy)]Cl,2bCl (dtBubpy is 4,40-di-tert-butyl-2,20-bipyridine) (Figure
4). In degassed MeCN the complexes Na1a and1bCl exhibited vibronic ligand-centered (3LC) emission profiles, respectively, atλ
PL= 472 nm andλPL= 458 nm with photolumines- cence quantum yield ofΦPL= 70% andΦPL= 38%, and photoluminescence lifetimes ofτPL = 4.0 µs andτPL = 36.7 µs. Similarly, complex Na2a exhibited a vibronic 3LC emission at λPL= 448 nm with a ΦPL of 70% and aτPL = 4.1 µs, while complex2bCl showed a broad mixed metal-to-ligand and ligand-to-ligand charge transfer (3MLCT/3LLCT) emission centered at λPL = 586 nm with a lower ΦPL of 21% and a shorter τPL of 0.43 µs. The photoluminescence spectra of the soft saltsS1andS2exhibited a concentration dependence, which impacted the degree of emission quenching of the anion by the cation via Dexter energy transfer. Taking S2as an example, at a relatively low concentration of 10−5 M its luminescence profile was dominated by the emission at λPL= 448 nm, characteristic of the blue-emitting anion 2a. This high-energy emission gradually decreased as a function of increasing concentration of S2 from 10−5 M to 10−3 M and, at a concentration greater than 10−3 M, only the orange emission exhibited by the cation2batλ
PL= 586 nm was observed. Based on a bimolecular quenching model, a quenching rate constantkq of 1.71 x
1010 M−1·s−1, close to the diffusion limit in acetonitrile (2 x 1010 M−1·s−1), was calculated. Similarly, De Cola and co-workers assembled the green-emitting anionic Ir(III) complex of composition NBu4[Ir(ppy)2(CN)2], NBu43a, with the yellow-emitting cationic Ir(III)
complex [Ir(dFppy)2(bpy)]Cl,3bCl (S3,Figure 5), and the blue-emitting anionic Ir(III) complex K2a with the orange-emitting cationic Ir(III) complex [Ir(ppy)2(bpy)]Cl, 4bCl (S4,Figure 5a).
Figure 5. a) Chemical structures of complex salts S3 (top) and S4 (bottom). b)
crystal packing of saltS3highlighting the channel running along the crystallographic c axis (top) and illustration of the dichloromethane solvent molecules (space-filling representation) inside the channel and pockets ofS3(bottom). The remaining solvent molecules are omitted for clarity. c)top: normalised emission spectra of dried S4(black line), S4with toluene intercalated (blue line), complexes2a (red line) and 4b(green line); bottom: normalised emission spectra of S4(black line) and S4intercalating anthraquinone (pink line). Partc)
is adapted with permission. Copyright 2010, Wiley-VCH.[72]
The X-ray single crystal structure of S4showed that this soft salt forms a 3D porous network where small solvent molecules such as dichloromethane, amyl acetate, ethyl acetate, diethyl ether, toluene or anthraquinone could be easily intercalated (Figure 5b). The emission properties of the single crystals of the microcrystalline salts S3 and S4 were investigated both under vacuum in the absence of guest molecule inclusion and after loading guest molecules into their networks. In degassed CH2Cl2, complex NBu43a emitted yellow light with a λPL at 564 nm while 3bCl emitted in the blue-green with a λPL at 502 nm. The emission of the crystal of S3 in its dry form exhibited a red-shifted emission atλPL = 591 nm. Analogously, the dried crystal of S4exhibited a red-shifted emission at λPL = 596 nm compared to the emission of both K2a and 4bCl, which exhibited, respectively, λPL = 554 nm and λPL = 460 nm. The red-shifted emission of both S3 and S4compared to the corresponding mononuclear complexes were attributed to strong π-π-interactions between the CˆN ligands of complementary iridium complexes of opposite charge present in the crystal networks, promoting Dexter energy transfer from the high energy anionic Ir donors 3a and 2a to the low energy cationic Ir acceptors 3b and 4b,
and additionally promoting exciplex formation and emission from correspondingly lower- energy excited states. Importantly, the emission properties of the crystals S3andS4could be efficiently modulated by trapping guest molecules within their porous networks. For example, the intercalation of toluene or anthraquinone intoS4 led to blue-shifted emission of the soft-salt, respectively, at λPL= 590 nm and λPL = 580 nm, compared to the dried crystal. An enhanced emission was observed when toluene was absorbed into S4due to the disruption of theπ-π-interactions whereas the emission was quenched when anthraquinone was intercalated within the crystal of S4as a function of the photoinduced electron transfer from the S4donor to the anthraquinone acceptor (Figure 5c).
Sandroni and Zysman-Colman[73] reported the first example of a three component heterometallic ion-pair assembly, S5 (Figure 6) involving two equivalents of complex NBu43a (Figure 5a) associated with the red-emitting [Ru(dtBubpy)3]Cl2 5bCl2 (Figure 6a). Upon photoexcitation into the ruthenium complex1MLCT absorption band in MeCN,
5bCl2 exhibited the characteristic broad3MLCT emission centered atλ
PL= 630 nm, which is complementary in color compared to the3LC emission exhibited by complex NBu43a (λPL = 477 nm in degassed MeCN).
Figure 6. a) Chemical structure of the soft salt S5 with its single crystal x-ray structure;b) normalised emission spectra of S5recorded in deaerated MeCN at 298 K at different concentrations (λexc = 390 nm). Insets are images of MeCN solutions of S5at different concentrations. Image b)is adapted from ref [73] with permission from The Royal Society of Chemistry.
Soft salt S5 nicely illustrates how the emission properties can be modulated upon changes in concentration and medium. At low concentration, the emission of S5is dominated by the structured 3LC emission of 3a. As the concentration increases, due to increased
efficiency of the Förster energy transfer from the anionic Ir(III) donor to the cationic Ru(II) acceptor, the emission profile ofS5resembles increasingly that of the lower-energy emission of the Ru(II) complex5b(Figure 6b). In contrast to the emission behavior observed for
S3andS4, the emission of S5did not involve either excimers or exciplexes. Further, as a function of the solvent polarity, the electrostatic interaction of the cation and anion could be modulated and therefore the magnitude of the energy transfer between the two. Solvents of high polarity, such as DMSO, MeOH or EtOH, solvated strongly the ions, leading to their weak association, poor energy transfer and an emission dominated by3a. Less polar solvents such as CH2Cl2 or MeCN promoted the formation of intimate ion pairs, resulting in shorter distance between the Ir and Ru ions and more efficient energy transfer, and an emission profile that resembles that of 5b.
Soft salt S5exhibited a significantly enhanced ECL (electrochemiluminescence) signal at similar energy to5b.[74] This observation is rather unusual considering that for the vast majority of multichromophoric species, the ECL signals could be addressed at different potentials and thus produce multiple emissive readouts.[75–77] The ECL efficiency of S5
was determined to be 2.51%, which is intermediate between the ECL efficiencies of the ionic components3a (2.83%) and 5b(2.14%). The higher ECL efficiency forS5compared to
5bis due to the [Ir]·--[Ru]·+ annihilation process, where [Ir]·- acts as a co-reactant and thus reduces the energy required to emit light from the excited [Ru]*.
Godbert and co-workers[78] synthetised green-emitting anionic Ir complexes bearing an unsual bidentate orotate dianion as the ancillary ligand (NBu44a and NBu44binFigure 7a). These complexes exhibited high photoluminescence quantum yields of ΦPL = 69%
and ΦPL = 58%, respectively, with emission maxima, respectively, at 530 nm and 536
nm. When NBu44a and NBu44bwere respectively assembled with the cationic complex [Ir(ppy)2(py-am)]Cl (6bCl in Figure 7a, py-am is 2-pycolylamine), which itself shows an emission at λPL = 490 nm with aΦPL of 52%, highly emissive soft salts, S6a andS6b
(Figure 7a) were obtained. S6aand S6bare unusual examples of soft salts in which the cation (6b) is the donor unit, while the anions (4aand4b) are the acceptors. The emission of S6a andS6b involved contributions from both ions withλPL, respectively, at 480, 525 nm and 486, 532 nm and highΦPL values of 81% and 83%, respectively.
Figure 7. a) chemical structure of soft saltsS6a andS6band b)chemical structure of the white-emitting soft salt S7. Inset is image of air-equilibrate CH2Cl2 solution of
S7. The inset image is adapted from ref [79] with permission from The Royal Society of Chemistry.
A white-emitting soft salt was obtained by Stagni and co-workers[79] by assembling the blue-emitting anionic Ir(III) complex NBu45a (Figure 7b), bearing dFppy as CˆN ligands and 4-benzonitrile tetrazolate as ancillary ligands, with the red-emitting [Ir(ppy)2(ptz- Me)]Cl complex (ptz-Me is 5-methyl-2-phenyltetrazole) (7bCl, Figure 7b). In aerated CH2Cl2, complex NBu45a exhibited a structured emission profile typical of emission from a 3LC/3MLCT state with λ
PL at 462 nm and 492 nm and aΦPLof 3.3%. Complex 7bCl,
by contrast, exhibited a broad emission profile with λPL = 686 nm and a ΦPL of 2.7%.
As both the anionic and cationic complexes 5a and7bcontributed almost equally to the emission of S7in aerated CH2Cl2, and energy transfer between the two ions played a minor role, an almost pure white-light (CIE: x = 0.3288, y = 0.3284) with λPL at 460 nm, 490 nm and 680 nm and aΦPL of 2.8% was emitted by the assembly. The poor energy transfer
in S7 is probably due to the presence of the bulky 4-benzonitrile tetrazolate in 5a that restricts its interaction with the cation7b.
In the context of electroluminescence devices, neutral Ir complexes have been extensively used in Organic Light-Emitting Diodes (OLEDs),[80–82] whereas cationic Ir(III) complexes have been explored more explicitly as emitters in Light-Emitting Electrochemical Cells (LEECs).[83, 84] OLED and LEEC technologies will be further discussed in Chapter 2.
Covalently linked dinuclear iridium complexes have been scarcely investigated as emitters in solid-state lighting, with only one example available for use in LEECs[85] and few for OLEDs.[86–92] However, iridium-based soft salts have demonstrated reasonable performance as emitter materials in OLEDs. The main advantage of using soft salts in lighting devices relies on the capacity to introduce two phosphorescent centers in one complex ion, while controlling the intermolecular separation of the two metal centers through non-covalent interactions. The first examples of iridium soft salts used in OLEDs were reported by Thompson and co-workers.[71] In OLED technologies the two most important parameters to evaluate the device efficiency are the External Quantum Efficiency (EQE), which defines
the ratio of the number of photons emitted from the device to the number of electrons passing through the device, and the luminance, which defines the intensity of light emitted from the OLED per unit area in a given direction. It is worth mentioning that OLED devices fabricated by using the green-emittingfac-[Ir(ppy)3] complex exhibited a high EQE
of 21.3% and a luminance of over 23000 cd·m−2. The first use of fac-[Ir(ppy)
3] in OLEDs was reported by Thompson and co-workers in 1999,[93] and since then it is still among the best performing metal complexes used in OLED devices.[2] Notably, OLEDs fabricated by using S2(Figure 4) as the emitting material exhibited an external efficiency, EQE, of 4.7% and a luminance of over 7428 cd·m−2 (λ
PL = 586 nm). The good performance of the OLED could be directly linked to the suitable alignment of the HOMO-LUMO levels of the two ionic components of the soft salt. Unfortunately, the relatively low ΦPL of 18%
exhibited by S6in thin films limited the efficiency of the device. WhenS6was used as the emitter in a single-layer LEEC, the device failed to turn on under voltages ranging from 2.5 V to 7 V, a result of the poor ionic mobility of the ions of S6.
Dumur, Mayer and co-workers[94] also explored the use of a soft salt emitter in OLEDs, the composition of which consisted of the anionic NBu4[Ir(dFppy)2(NCS)2], NBu46a, with the cationic complex [Ir(ppy)2(non-bpy)]PF6 8bPF6 (Figure 8a, non-bpy is 4,40-dinonyl-
2,20-bipyridine). The nonyl chains were introduced to 8bPF6 to enhance the solubility
of the resultant S8 in organic solvents and thus improve the thin film morphology in solution-processed devices. Soft saltS8exhibited luminescence centered at approximately 550 nm in CH2Cl2 solution, thin film and in electroluminescent devices. The OLEDs fabricated withS8as the emitting material exhibited a low external quantum efficiency of 0.66% and a luminance of 1114 cd·m−2 (λ
EL= 553 nm).
Two iridium soft salts based on ion-paired dinuclear cationic, and mononuclear anionic complexes were reported by Mayer and co-workers[95] and investigated as emitting materials in OLEDs (Figure 8b). The unit bridging the two iridium centers in the dinuclear cationic complexes consisted either of a carbazole derivative for 9bPF6 or a phenylene group in the case of 10bPF6. Both dinuclear complexes 9bPF6 and 10bPF6 exhibited lowλPL in degassed CH2Cl2 of 9.1% and 15.0%, respectively, attributed to intramolecular quenching between the two iridium luminophores. As the phenylene bridge favored the communication between the two iridium centers, the electroluminescence performance usingS10was poorer than that using S9 with respective maximum peak current efficiencies of 0.06 and 0.44 cd·A−1 and maximum brightness of 101 and 1022 cd·m−2.
Figure 8. Chemical structures of iridium soft salts used in OLEDs. a) S8andb) S9
(top) and S10(bottom).
Intracellular pH is a crucial parameter associated with cellular behavior and pathological conditions such as cell proliferation, apoptosis, drug resistance, enzymatic activity and ion transport.[96] Abnormal cellular pH is an indicator of inappropriate cellular function, which is associated with many diseases such as stroke, cancer, and Alzheimer.[97] It is thus pivotal to monitor pH alteration in biological cells and tissues to understand physiological and pathological processes.[98] In this context, Wong, Zhao and co-workers[99] recently illustrated that soft salts can also be rationally designed to act as efficient probes for lifetime imaging of intracellular pH. Indeed, they designed the soft salt S11(Figure 9a) formed by the assembly of the cationic complex 11bCl, bearing a qpy (qpy = 4,40:20,200:400,4000-
quaterpyridine) as the ancillary ligand, with the anionic complex NBu42a (Figure 4). Specifically, complex 11bCl is pH sensitive as the protonation or deprotonation of its distal
pyridine units gives rise to changes in the emission of S11, which can be easily detected by both steady-state and time-resolved luminescence spectroscopy. The cationic complex
11bCl showed a broad orange-red emission in deareated MeCN centered atλPL = 625 nm, which decreased dramatically in intensity with decreasing pH (Figure 9b). By contrast, the structured blue-emission exhibited by the anionic counterpart NBu42a (λPL = 451, 475 nm) was not affected by changes in pH.
Figure 9. a) Design concept of the ratiometric design probe and chemicals structures of 2a, 11band soft salt S11. b)changes in the phosphorescence emission spectra of 11b
(2.0 x 10−5 M) in the pH range of 2.03 – 7.94 in MeCN/buffer (1:9, v/v). c)changes in the phosphorescence emission spectra of S11 (2.0 x 10−5 M) in the pH range of 2.03 – 7.94 in MeCN/buffer (1:9, v/v). d)phosphorescence lifetime images of S11 in living Hep-G2 cells (incubated at 37◦C for 1h) at different pH values. Images b),c)and d)are adapted from
ref. [99] with permission from The Royal Society of Chemistry.
The phosphorescence spectral changes of S11 at different pH values are illustrated in Figure 9c. Enhanced phosphorescence at λPL = 625 nm was observed at basic pH, while at acidic pH the emission of 11b was quenched due to the protonation of the qpy
ligand. Thus, at basic pH the emission of S11 was dominated by the orange emission of 11b, the result of efficient energy transfer from2a, while at acidic pH the emission of
S11 was dominated by the structured blue emission of the donor anion. Such a change in phosphorescence from blue to orange with increasing pH values was only observed when 2a
was combined with the cation11b, the emission intensity of which is highly pH dependent. The change in photoluminescence from blue to orange with increasing pH was also detected using confocal luminescence microscopy after incubation of S11 in living HepG-2 cells (Figure 9d).
Similarly, Zhao and co-workers[100] used an ion-paired iridium complex (S12, Figure 10a), composed of a cationic complex functionalised withα,β-unsaturated ketone moieties (12bPF6), and the anionic complex NBu43a(Figure 5a), for ratiometric and time-resolved luminescence sensing and imaging of intracellular biothiols cysteine and homocysteine. These analytes were chosen as they participate in the process of cellular growth in living cells. The sensing capacity of S12 towards thiol analytes is due to their rapid Michael addition with the α,β-unsaturated ketone moieties of 12b thereby promoting a strong enhancement of the emission of the cation acceptor at λPL = 560 nm, while only slightly influencing the photophysics of the donor complex at λPL = 485, 505 nm (Figure 10b,c).
Figure 10. a)chemical structures of3a,12band soft saltS12and sensing mechanism of S12. b)changes in phosphorescence emission spectra of S12 in MeCN/H2O (3:2, v/v) with various amount of cysteine added (from 0 to 2 equivalents). Insets are images of S12
without cysteine (left) and with two equivalent of cysteine (right). c)titration curve plotted with emission intensity at 560 nm over that at 485 nm as a function of cysteine equivalents. Images b)and c)are adapted from ref. [100] - published by OSA publishing.
Schanze and co-workers[101] reported an ion-paired assembly between the anionic poly(phenyleneethynylene) electrolyte (PPESO−
3) and the cationic Ir(III) complex 4bCl (Figure 5a). Upon excitation of 4b-PPESO−
3 in methanol at 355 nm, the emission of PPESO−
3 at λPL = 450 nm was completely quenched as a result of Dexter energy transfer from PPESO−
3 to 4b with a calculated quenching constant (kq) of around 105
M−1. A weak phosphorescence from 4bwas observed at λ
PL = 610 nm. Subsequent to the Dexter energy transfer, ultra-fast back-transfer from the triplet state of 4blocated at 2.25 eV to the triplet state of PPESO−
3 located between 2.0 – 2.2 eV was detected by transient absorption spectroscopy. By contrast, back-energy transfer was not observed when PPESO−
3 was assembled with the Ir(III) complex of composition [Ir(hqx)2(bpy)]Cl (hqx is 2,3-diphenylquinoxaline), which possesses a triplet state at 1.96 eV, which is too low in energy to sensitise the formation of the triplet state of PPESO−
3. This study demonstrates that Ir(III) complexes with appropriate triplet energy can serve to sensitise the triplet state in conjugated polymers.